This invention relates generally to a voltage sensing system and particularly, but not by way of limitation, to a voltage sensing system with input impedance balancing for electrocardiogram (ECG) sensing applications.
When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body""s circulatory system. The body""s autonomous nervous system generates intrinsic electrical heart activity signals that are conducted to atrial and ventricular heart chambers on the left and right sides of the heart. The electrical heart activity signals trigger resulting heart contractions that pump blood.
The intrinsic electrical heart activity signals can be monitored to provide an electrocardiogram (ECG) signal to a physician, clinician, diagnostician, or researcher to obtain information about heart function. In one such technique, a first external skin patch electrodes is adhesively affixed to the patient""s right arm. A second external skin patch electrode is adhesively affixed to the patient""s left arm. An instrumentation amplifier is used to detect the electrical heart activity signals at the first and second electrodes. The instrumentation amplifier outputs an ECG signal based on the difference of the signals at the first and second electrodes.
If no further electrodes are used, the ECG signal obtained between the first and second electrodes is typically severely degraded by common-mode (CM) noise signals, such as 60 Hertz or other environmental noise signals that are present at both of the first and second electrodes. Common-mode noise problems generally result even if a high-quality instrumentation amplifier is used. Skin-electrode interface impedance differences between the first and second electrodes contribute to such common-mode noise problems. Differences in skin-electrode interface impedances result from differences in body morphology, adhesion of the electrode, perspiration by the patient, etc. Because of the high input-impedance of the instrumentation amplifier, even small differences in the skin-electrode impedance (e.g., 10 kilo-ohms) can result in a common-mode noise signal amplitude that exceeds the amplitude of the desired ECG signal.
One technique of reducing the common-mode noise signal is to attach a third electrode, such as at the patient""s right leg, for use in a feedback arrangement. The third electrode is driven by an offsetting common-mode signal to cancel a portion of the unwanted common-mode noise signal. However, this technique is inconvenient for the physician, because it requires attachment of the third electrode to the patient. This increases the complexity of the medical procedure. In a medical emergency, for example, such increased complexity is highly undesirable. Thus, there is a need for improved ECG measurement techniques providing adequate common-mode noise immunity without relying exclusively on attaching additional electrodes to the patient.
The present system provides, among other things, a voltage sensing system with input impedance balancing for electrocardiogram (ECG) sensing or other applications. The present system allows sensing of ECG or other input voltage signals and reduces sensing of unwanted common-mode noise signals. The present system is capable of use with two electrodes, while still providing good signal-to-noise characteristics.
According to one aspect of the present system, signals are received at first and second electrodes or terminals, each having an impedance associated therewith. An effective impedance associated with the second electrode is adjusted based on an effective impedance associated with the first electrode. In one embodiment, an impedance circuit adjusts the effective impedance associated with the second electrode based on difference and common mode signals obtained from signals at the first and second electrodes. As a result, signals associated with each electrode undergo a similar degree of gain/attenuation and/or phase-shift. This reduces common mode noise and enhances the signal-to-noise characteristics of a desired ECG or other output signal, without requiring the use of more than two electrodes. Thus, in an ECG signal acquisition application, the present system enhances the noise immunity of the ECG signal without increasing the complexity of the associated medical procedure. Other aspects of the invention will be apparent on reading the following detailed description of the invention and viewing the drawings that form a part thereof.
According to another aspect of the present system, buffer amplifiers are added to the front end of the input circuitry, instead of compensating skin-electrode impedance mismatches at the input, to circumvent the problems associated with high capacitance cables,. This allows the ECG system to always present constant input impedance at the patient electrodes. This also permits higher capacitance values on the input without swamping out the impedance balancing range.
Another illustrative embodiment includes a switch to control flow of right leg (RL) signal (coming from the third electrode) into ECG cable to selectively turn-off the RL signal, when the RL electrode is not in use, to prevent the RL signal from entering the ECG cable for optimum performance (the standard industry ECG cable incorporates 4 input cables, and one RL output cable with a single connector connecting to the ECG system). Generally there is no means available to disconnect the RL cable, when the right-leg drive is not in use).
Another illustrative embodiment includes an automatic gain control module between the common-mode input and the impedance circuit to maintain a constant transient response time over a wide range of input noise levels.
Other aspects of the invention will be apparent on reading the following detailed description of the invention and viewing the drawings that form a part thereof.